Myocardial infarction (MI) occurs in almost a million people each year in the United States, where coronary heart disease is the leading cause of hospital admissions. According to the joint American College of Cardiology and European Society of Cardiology consensus document concerning the redefinition of MI, the diagnosis of MI is often based on cardiac biomarkers and ECG changes. However, biomarkers are only elevated for 4 to 10 days after an acute event. Thus, biomarkers are not useful for the diagnosis of subacute or chronic MI. The ECG also has limitations: Q waves that form the fundamental basis of the diagnosis of chronic MI may be absent or, if initially present, may disappear at a later time point.
Cardiovascular magnetic resonance (CMR) imaging is a highly attractive modality for the assessment of myocardial infarction and viability because of high spatial resolution and accuracy. However, practical drawbacks have in the past limited the impact of such technology for general clinical use. For example, standard CMR is generally more complex as compared to some imaging modalities. Patient and protocol setup times have been generally longer, and multiple breath-holds and longer scanning times have been necessary. These conditions can limit clinical throughput and the types of patients that can be scanned, and may also increase the complexity and length of CMR training.
In view of the importance of this area of investigation to societal health, much work has been done in the past to develop imaging techniques capable of detecting infarcts and determining viability.
For example, work at Northwestern University by Dr. Raymond J. Kim and others resulted in U.S. Pat. No. 6,205,349 entitled “Differentiating normal living myocardial tissue, injured living myocardial tissue, and infarcted myocardial tissue in vivo using magnetic resonance imaging”, describing techniques for distinguishing between normal, injured but living, and infarcted myocardium using MR imaging. Because they solve many of the challenges described above, such delayed contrast-enhancement CMR techniques have become the gold standard for imaging myocardial infarction. Such delayed enhancement images exhibit excellent contrast between normal and infracted myocardium due to nulling of normal myocardium. The current clinical standard is a so-called segmented IR-Turbo Flash technology that acquires data during a breath hold of typically 10 seconds or less. Furthermore, using such delayed contrast enhancement techniques, myocardial infarction can be detected rapidly by subsecond delayed contrast-enhancement CMR during free breathing with high accuracy. The clinical implication is that delayed contrast-enhancement CMR can approach a quick “push-button” technique with the ability to scan a wide range of patients, including those who are more acutely ill, those with dyspnea, or those unable to undergo a prolonged examination. Moreover, clinical throughput could be increased multifold. See e.g., Sievers et al, “Rapid Detection of Myocardial Infarction by Subsecond, Free-Breathing Delayed Contrast-Enhancement Cardiovascular Magnetic Resonance”, Circulation 2007;115;236-244 and other articles by Drs. Kim, Judd and/or Rehwald.
Such known prior art delayed enhancement MRI pulse sequences can deliver images in which viable myocardium (living heart tissue) appears dark and infarct and blood appears bright. For example, a frequently used exam in cardiovascular MRI is called “viability imaging” or “myocardial delayed enhancement.” For this test, a MR contrast agent is injected into the patient intravenously while the patient is lying in the MRI scanner. After about 10 minutes, the contrast agent has distributed throughout the patient's body including the heart. In the heart, this agent accumulates primarily in dead heart cells and in scar, both known as infarcted territory (such cells have died e.g., due to a prior heart attack—infarct). As the contrast agent regionally alters the magnetic properties of the heart tissue and as it primarily accumulates in the dead cells, it is possible to visualize regions of dead cells with MRI.
One so-called “pulse sequence” provided by software running on a MRI scanner that can be used to provide such imaging is called Inversion Recovery Turbo Fast Low Angle Shot (IR−TurboFlash). This is the vendor acronym of Siemens Medical Solutions. Other vendors such as GE Medical Systems and Philips have similar products with different acronyms. Such methods reliably deliver images in which viable myocardium (living heart cells) appears dark, and infarct (dead heart cells or scar) and blood appear bright.
Such techniques as described above are sufficient in many cases to detect infarcts and help physicians determine myocardial viability. For example, the identification of large areas of dysfunctional but viable myocardium predicts situations where revascularization is likely to improve functional class, augment regional and global LVEF and increase survival. Conversely, the presence of predominant myocardial scarring predicts increased operative mortality and the absence of these salutary effects.
While such “bright blood” imaging and analysis techniques have been successful, it has become evident that in patients with small subendocardial infarcts (infarcts located at the inner side of the heart wall adjacent to the blood pool), the bright blood can sometimes obscure the bright infarct. Small subendocardial infarcts are sometimes difficult to detect as they may have a similar signal intensity (T1 values) as the blood pool. Therefore, small subendocardial infarcts are often hard to detect in standard delayed enhancement images. The infarct may be missed or its size may be underestimated. Ideally, a viability sequence would have excellent contrast between infarct and both normal myocardium and the blood pool. Techniques providing images in which blood appears dark/black while leaving the infarct bright and normal myocardium dark have been highly sought after.
So-called “dark blood” or “black blood” angiography MRI pulse sequences are known. Such techniques make flowing blood appear dark or black in the image and make stationary blood or tissue appear to be bright in the image. In one such type of “black blood” pulse sequence, early echoes are more heavily proton density weighted than later echoes (the later echoes can be more T2 weighted). Depending on the exact sequence implementation, the obtained images can be more proton-weighted or more T2-weighted.
Also, the idea of combining a slice-selective with a non-selective inversion pulse has been used for several years for acquiring black-blood images. However, generally the two pulses are played immediately after one another. A known technique called “Black-Blood HASTE” is usually used without the presence of contrast agent in the blood pool, and it only nulls (makes black) blood, not normal myocardium.
For example, it is generally known that nulling two T1-species can be achieved by a timed combination of two non-selective inversion pulses. However, due to their similar T1 values, the contrast between infarct and blood is still small.
Additionally, techniques are known that decouple blood preparation from tissue preparation by use of a non-selective inversion followed by a slice-selective “re-inversion” followed by image acquisition (“Black Blood HASTE”). Such techniques work well because there is sufficient time for blood exchange and preparation and readout occur when the heart is in nearly the same position (before contraction and during mid to late diastole, respectively). However, a recurring problem has been that the standard classic double-inversion “dark blood” approach does not work in conjunction with delayed enhancement since the contrast agent is present and since only one T1 species is nulled and so cannot be used for viability imaging without adding further preparation pulses. There are therefore challenges associated with using the classic approaches for dark blood viability. For example, the classic dark blood preparation does not provide T1-weighting of the tissue. An additional IR pulse would need to be played before or after to double-IR dark blood preparation to get dark blood delayed enhancement images. Generally, the simultaneous nulling of blood and normal myocardium would be extremely difficult as there may be insufficient blood exchange between double IR preparation and image readout.
Dark blood delayed enhancement techniques that could image blood and normal myocardium as dark/black while leaving the infarct bright would be clinically useful. For example, it would be desirable to provide techniques which would:                Null blood and normal myocardium at the time of readout;        Play slice-selective preparation before systolic contraction (blood exchange, slice position);        Provide image readout late enough (blood exchange, similar slice position as during slice-selective preparation); and        Allow a non-selective preparation to be played out at any time.        
The idea of nulling more than one type of tissue per se is known. However, to our knowledge, it has not generally been used in a combination of early slice-selective preparation, blood exchange of the heart, and late non-selective preparation in the presence of MR-contrast agent to acquire “black-blood viability” images (note that in this context, “viability” refers to the property of heart tissue, myocardium, of being alive, “viable”, or dead, “non-viable”).
We have now developed a new “dark blood” or “black blood” myocardial viability delayed enhancement imaging technique which can obtain dark blood delayed enhancement images by a timed combination of a selective preparation after the cardial R-wave and a later, non-selective inversion. Such MRI sequences can aid the detection of small subendocardial infarcts.
One exemplary illustrative non-limiting implementation combines an early slice-selective magnetic preparation of heart tissue, followed by a particular calculated delay to allow blood exchange of the heart, followed by late non-selective inversion (e.g., in the following heartbeat). The timed combination of slice-selective and non-selective preparation decouples the infarct-curve from the blood-curve and enables greater image contrast than is possible for example using two non-selective preparations.
In one exemplary illustrative non-limiting implementation, to make both blood and non-infarcted myocardium appear black in the image, a preparation is used to cause the relaxation curves of both T1-species to simultaneously cross the zero-line (“be nulled”). In one exemplary illustrative non-limiting implementation, the timed combination of slice-selective and non-selective preparation decouples the infarct from the blood curve and enables good image contrast.
In one exemplary illustrative non-limiting implementation, the time from the nsIR (non-selective inversion recovery) to the center of K-space is chosen to null blood and only depends on its T1. The time between the SSSR (slice-selective saturation recovery) pulse and nsIR pulse is set to null normal myocardium when blood is nulled.
In one exemplary illustrative non-limiting implementation, contrast between infarct and blood is improved at the expense of a lower infarct signal-to-noise ratio. Gradient, turbo-spin echo (tse) and SSFP readouts can be used. The tse readout results in a high signal to noise ratio and better blood nulling.